1) Convenient cave collapse
2) Convenient force field
3) Convenient threat from the other end of the tunnel

The only reason we have not been attacked yet is probably because tal is giving us a second to prepare our defence. ( I think. )

If this were a trap, we would have been attacked after we first passed the lights. As is is, we passed the lights, stopped for a cave-in, and then left.
Now, we see movement near the cave in. Apparently those things came out to investigate the collapse.

As the last of your group passes the first set of vertical, glowing bar lights, the two on the end shift their color to a bright cyan. A blue, shimmery field appears between them, and you wonder if it's a shield. It might be. You're not sure. It doesn't look all that solid. It's more of a gossamer thing. You do see something moving near the rubble pile up ahead - something shadowy. You get the feeling it might be hostile.

"Guys, I think we might have company," you warn, trying to get a better view. But it's no good. It's simply too dark.

As the last of your group passes the first set of vertical, glowing bar lights, the two on the end shift their color to a bright cyan. A blue, shimmery field appears between them, and you wonder if it's a shield. It might be. You're not sure. It doesn't look all that solid. It's more of a gossamer thing. You do see something moving near the rubble pile up ahead - something shadowy. You get the feeling it might be hostile.

"Guys, I think we might have company," you warn, trying to get a better view. But it's no good. It's simply too dark.

Thats a quote from Dak's post.

You get the feeling it might be hostile.

Hint hint?

"Might" does not mean that they're hostile, Trigger.

I think we should step back and look at this situation instead of continuing this silly argument.
As Fawkes pointed out on IRC, the lights turned cyan when Unit got to them. That's also when the blue field appeared.
When we passed them, they turned red. It appears that they may be scanning us somehow. It's fairly obvious that they know what we are.